Heat pipes used in displacement ventilation
Ventilation is the introduction of outside (primary) air
to a space while polluted internal air is extracted.
Traditional methods of ventilation (mixing ventilation)
involve the introduction of a mixture of treated outside
air and recirculated indoor air at high level outside
the occupied zone. This air will be cold at typically 12
to 15°C and supplied at relatively high velocity. Air
needs to be cold so that when combined with the
relatively high air volume it will offset the internal
heat gain, and at relatively high velocity so that the
air mixes with the indoor air effectively outside of the
occupied zone. The supply air will typically leave the
supply diffuser at 1 to 1.5m/s.
Conventional air conditioning systems can be either a)
all air systems i.e. variable air volume, constant air
volume or b) air-water systems using fan coil units. In
all air systems space cooling is provided by supply air
from the central AHU whereas an air-water system
provides outside air from the central AHU and local
cooling via the chilled water fed to the FCUs. The
primary air can be mixed with room air at the FCU, or
alternatively, fed via a separate supply. FCUs can
provide both sensible and latent cooling to a space,
whereas in all air systems all the latent cooling must
take place at the central AHU. Displacement
ventilation/chilled ceiling systems are a special case
of an air-water air conditioning system where the space
cooling equipment must do no latent cooling.
Displacement ventilation differs from mixing ventilation
as follows: Air is introduced at low level directly into
the occupied zone, at relatively warm temperatures,
typically 18 to 19°C and at low velocity. Supply air
volumes are low and only include treated outside air.
The theory behind displacement ventilation is:
Air around 3°C colder than the space is supplied at
floor level into the occupied zone. Being colder than
the space air it is more dense and displaces the space
air above it. When the cool, dry displacement air
contacts sources of heat and moisture it is warmed and
becomes more humid, forming plumes of low velocity air
around people and equipment, cooling them directly and
transferring their heat and moisture upwards.
It is suggested that mixing ventilation is responsible
for distributing contaminants associated with the room
fabric and occupants whereas the plumes of warm air
typical of displacement systems are unpolluted and
result in contaminants collecting at ceiling level where
they are extracted. Accordingly, displacement
ventilation results in a healthier environment than
mixing ventilation. Because air is introduced at floor
level it must be at a comfortable temperature i.e. 18 to
19°C. It must also be supplied at low velocity not to
create uncomfortable draughts. When the low supply air
volume is combined with the relatively high supply
temperature the amount of cooling that can be achieved
using only displacement air is limited and typically
accounts for only 25% of the total space cooling load.
Cooling provided by the ventilation air is referred to
as primary cooling. A further source of cooling
(secondary cooling) must be provided. When using
displacement ventilation the low air velocities give
rise to stratification, warm polluted air gathering at
ceiling level. To counteract this, the displacement
ventilation system always combines with a chilled
ceiling and high level extract system.
Chilled Ceiling
A chilled ceiling can consist of chilled beams, chilled
panels or a combination of both. Beams are normally
found around the perimeter where solar gains are high,
panels can be used in the interior of the zone where
heat gains are low. Beams can be passive, relying on
warm air to convect over the chilled fin surfaces, or
active using low powered fans or induction units. As the
air is cooled by the beams/ceiling it must not come into
contact with surfaces below the dewpoint of the air. If
it did then moisture would start to condense onto the
ceiling and eventually drip into the room.
For the chilled ceiling to achieve the necessary cooling
it must be fed with chilled water at a sufficiently low
temperature, while still being above the dewpoint of the
air which comes into contact with it. The water to the
chilled ceiling is typically supplied at between 14 and
16°C. Assuming that the dewpoint of the displacement air
increases by 1°C as it picks up moisture between the
floor and ceiling, it is typically supplied at dewpoint
of 2°C below the temperature of the chilled water
feeding the ceiling.
Application of Heat Pipes
Displacement ventilation/chilled ceiling systems work
against almost identical sets of conditions, with
outside air volume being determined by size and usage of
the space or level of occupancy. These conditions are:
Supply air volume: Typically 20 litres/s/person
Supply air temperature: 18°C
Supply air dewpoint: 12°C
Chilled beam water temperature: 14°C
To achieve the above supply conditions , the outside air
must be overcooled to remove moisture, before being
reheated to the comfortable supply temperature of 18°C.
Conventionally this is achieved by oversizing the
cooling coil to reduce the air to 12°C in a saturated
state then reheating to 18°C using LPHW, steam or
electric reheat.
This process is energy consumptive. By wrapping heat
pipes around the cooling coil both total cooling and
reheat requirements will be reduced or eliminated.
‘Wrap-around’ heat pipes are used, fitted around
conventional cooling coils. Heat pipes are extremely
effective heat conductors due to their internal
construction. They absorb heat at their warmer end and
transfer it to their cooler end with only negligible
temperature differences along their length. Visually,
heat pipes mimic the cooling coil consisting of bundles
of heat pipe tubes expanded into continuous fins. The
face size of the heat pipe will match that of the
cooling coil.
The wrap-around heat pipe absorbs heat from air entering
the cooling coil and transfers it to the leaving air .
This process effectively precools the air prior to the
cooling coil and reheats the air after leaving.
Precooling and reheat effects are equal and eliminate or
reduce the conventional costly overcooling and
reheating. Typical design conditions for displacement
ventilation heat pipe system are:-
Outside air: 29.0°C dry bulb/20.0°C wet bulb
Air off Precool: 23.0°C/18.0°C
Air off Cooling Coil: 12.0°C/11.8°C
Air off Reheat: 18.0°C/14.2°C
These conditions are plotted on a psychometric chart
Based on the above conditions, savings per cubic metre
outside air accrued through adding heat pipes are:
Precool saving: 7.2kW
Reheat saving: 7.2kW
At design conditions the heat pipe provides all
necessary reheat. Below this condition, supplementary
reheat is needed - either a conventional reheat coil or
a heat recovery heat pipe between supply and extract
decks, utilising heat from the extract air.
Heat recovery heat pipes will typically be a straight
heat pipe with its lower end in the warm extract air and
its upper end in the cool supply air. This will then
transfer waste heat from the extract air to the supply
air to provide the necessary reheat free of charge.
Figure 4 and 5 (see below) gives details of AHUs with
and without the heat recovery device.
Concluding remarks
1. Displacement ventilation is more energy efficient
than conventional systems as it involves treating
relatively low volumes of only outside air.
2. Displacement ventilation directly cools people and
equipment, the warm, moist plumes of air gathering at
the ceiling.
3. Displacement ventilation systems are healthier than
mixing ventilation systems as contaminants are not
distributed in the space but concentrated at ceiling
level were they are extracted.
4. Chilled ceilings are used to supplement the cooling
available from the displacement air.
5. Humidity of the displacement air must be controlled
to prevent the ceiling from dripping.
6. Outside air must be treated to remove moisture and
provide a comfortable temperature for supply into the
occupied zone. Heat pipes allow the most energy
efficient means of conditioning the
outside air in this fashion.
7. Both ‘wrap-around’ heat pipes and heat recovery heat
pipes can be utilised to give a complete solution
irrespective of the external conditions and without the
need for costly conventional reheat.
8. Heat pipes are external energy free, using only the
refrigerant contained within the heat pipe circuits to
achieve the necessary pre-cool and re-heat actions.
9. Heat pipes have no moving parts.
Email:
spc@spcoils.co.uk
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